Magnetic bubble memories are well known in the art. One mode of operating such memories is called the "conductor-access" mode. Such a mode employs electrical conductor patterns overlying the layer in which bubbles move. Currents impressed in the conductors generate localized field gradients which move the bubbles.
An alternative mode of operating a bubble memory is called the "field-access" mode. The movement of bubbles in this mode is responsive to a magnetic field reorienting in the plane of bubble movement. Localized field gradients necessary for bubble movement are due to patterns of magnetically soft material in a layer coupled to the bubble layer. The in-plane field generates changing pole patterns in the magnetically soft material and the pole patterns cause bubble movement.
In both the conductor-access and the field-access arrangements, a magnetic bias field antiparallel to the magnetization of a bubble maintains the bubbles at some nominal operating diameter. Since bubble layers are characterized by a uniaxial anisotropy normal to the plane of bubble movement, the direction of the bias field is normal to that plane also.
In a field-access arrangement, in addition to the bias field magnet, coils are necessary to generate the inplane field as is well known. These coils have to be quite precise in order to generate a sufficiently uniform field over an entire bubble chip (or chips) encompassed within the confines of the coils. Because the in-plane field is generated, typically, by two coils driven in quadrature, one coil is placed within another leading to considerable constraints on coil design.
One problem in coil manufacture is that the strands of the outer coil for a field-access bubble memory migrate uncontrollably during the requisite molding operation which follows placement of the coils. Another problem is that multiple chip packaging requires close tolerances in coil winding which are expensive to achieve. Thus, the coils are expensive and packaging causes unwanted variations in the coils.